European patent application publication No. 393 917 discloses a process of removing excess ammonia by oxidizing the ammonia with molecular oxygen in the presence of a metal promoted zeolite. According to this publication, the oxidation of ammonia with molecular oxygen takes place in the presence of an iron promoted zeolite at temperatures above 500.degree. C. However, in the presence of the same catalyst the catalytic reduction of nitrogen oxides in the presence of ammonia takes place at about 350.degree. C., which is a much lower temperature.
It is an object of the present invention to provide a process of removing ammonia from an ammonia-containing gas mixture by means of oxidation which can be carried out at substantially the same temperature as catalytic reduction of nitrogen oxides.